Polyester compositions, such as polyethylene terephthalate or copolymers thereof (hereinafter collectively referred to as “PET”), are well known packaging materials. For example, in U.S. Pat. No. 4,340,721, a PET composition is used to manufacture beverage bottles and other containers (hereinafter referred to as “bottles”) by various molding methods.
In current practice PET bottles of the size and shape for most beverage applications are usually made by an injection stretch blow molding technique. Injection stretch blow molding has two main steps. First, the PET, in the form of granules, is melted in an injection molding machine and the melt injected into a cooled mold to form a precursor to the final bottle known as a “preform”. Commonly, the preform has a threaded neck with a shortened bottle body shape length about 8 to 20 cm and a material thickness between 3 mm and 6 mm. Second, the preform is transferred to a stretch blow molding machine where its external surfaces are reheated by infra-red (IR) lamps. Once the preform has reached a desired temperature, it is stretched and blown to form the final bottle.
The time it takes to reheat the preform is the rate-limiting factor for the overall process. The preform starts at ambient temperature and has to be heated to above the glass transition temperature of the polyester (generally to about 110° C.) so that the preform becomes sufficiently flexible to permit the stretch-blow step to work. In general, polyester polymers have a poor ability to absorb IR radiation. Hence, as well as extending the overall production time, the preform reheating step also requires a significant amount of energy. To address this problem, certain prior patents have taught that adding black materials and/or metal particles to PET compositions can reduce the time and energy required for reheating. Hence, prior patents teach adding carbon black (U.S. Pat. No. 4,476,272), iron oxide (U.S. Pat. No. 4,250,078), and antimony and other metal particles (U.S. Pat. Nos. 5,419,936 and 5,529,744) to reduce PET preform reheating time. Antimony metal particles were indicated as preferred because such particles preferentially absorb radiation at or near the infra-red wavelengths emitted by the IR lamps in most stretch blow mold machines, e.g., 500 nm to 2000 nm. Furthermore, as described in U.S. Pat. Nos. 5,419,936 and 5,529,744, antimony compounds are usually present in the polyester composition itself (as the catalyst for melt polymerization) and can be converted to antimony metal particles, with the desired IR absorption characteristics, by the addition of a reducing agent in the melt polymerization stage of manufacture.
Although PET has found widespread application for beverage bottles, the cost of raw materials for making PET is much higher than for some non-PET polymers. Therefore, the industry continually seeks to switch from PET to lower cost alternatives. Whilst seeking these alternatives, container manufacturers do not wish to invest substantial resources in new capital equipment to process a new polymer material, but would prefer to adapt their existing PET injection blow molding equipment for use with the new material.
One possible alternative to PET for use in injection stretch blow molding of beverage bottles is polypropylene. U.S. Pat. No. 6,258,313 teaches that injection stretch blow molding of a polypropylene preform is possible if the preform is heated simultaneously both from the outside and inside. Nevertheless, it has heretofore been more difficult to produce satisfactory beverage bottles from polypropylene by this method. First, polypropylene has a lower density and specific heat than PET and hence exhibits a significantly narrower processing window. Second, polypropylene suffers from the same limitations as PET in terms of its poor ability to absorb IR radiation. Furthermore, polypropylene generally has greater opacity than PET, which detracts from its aesthetic appearance. The industry thus continues to seek ways to improve the IR absorption properties of polypropylene such that it can be used to make beverage bottles on the same injection stretch blow molding equipment as PET and/or used to make other thermoformed articles.